Environmental pollution and resource shortage have become global problems in the 21st century. Meanwhile, they are drawing more and more attraction from human beings. The widely used synthesized molecular material made from petroleum has made great contribution to the development of human civilization. However, such materials are hard to be recycled, thus, nowadays having caused a severe problem called “white pollution.” On the other hand, since petroleum is a non-renewable resource, the increasing consumption of it forced human beings to face serious resource shortage. People are expecting biodegradable plastics (mainly made from aliphatic polyester) and especially hoping development and application of such biodegradable materials from renewable resources could significantly alleviate the two problems in the near future.
There are two ways to prepare polyester: ring-opening polymerization (indirect method) and direct polycondensation (direct method). The method of ring-opening polymerization is mainly applied to monomers such as cyclic lactone and cyclic lactide. The polyester prepared through ring-opening polymerization is characterized in high weight-average molecular weight, thermal stability, vitrification temperature and melting point, thus being widely used in industry. However, to prepare polyester by ring-opening polymerization requires technologies like preparation of cyclic monomer and refining and the like, which are complicated and time-consuming, therefore raising the cost of preparing polyester in the method of ring-opening polymerization.
Another common method is to synthesize polyester by direct polycondensation. Direct polymerization of polyester is a classical polycondensation reaction, and free monomers, water (or micromolecular alcohol) and oligomers are in a balance in the reaction system. In the formula for computing polymerization degree of polycondensation reaction:
  DP  =            K              n        w            DP represents polymerization degree of the reaction, K represents reaction equilibrium constant and nw, represents residual water.
According to the formula above, at a certain temperature, the only way to improve polymerization degree is to reduce the content of water or micromolecular alcohol since K is a constant, thus polyester with high weight-average molecular weight and melting point could be obtained. There are a lot of ways to remove water or micromolecular alcohol such as reducing pressure, raising temperature and prolonging reaction time. Generally, it is hard to prepare a polyester product with a high molecular weight by direct polycondensation. A polyester product with whose molecular weight over 50,000 is difficult to be prepared even taking no account of production efficiency, or prolonging time or adjusting temperature regardless of the cost. Aromatic polyester with a molecular weight about 10,000-40,000 is mainly synthesized in industry. To aliphatic polyester, if the molecular weight is lower than 50,000, it will lose its commercial value because of poor performance. Therefore, the method of direct polycondensation is barely used in industry to produce aliphatic polyester.
Direct polycondensation for polyester preparation mainly comprises four methods as below:
(1) Melt Polycondensation Method:
Melt polymerization is a polymerization reaction carried out at a temperature higher than the melting point of a polymer, which is a bulk polymerization. It has the advantage of being able to obtain pure product and requiring no medium separation. On the other hand, molecular weight of the product usually remains low since the more deeply the reaction processes, the harder the micromolecular by-product can be exhausted, therefore, the balance heads for the polymerization direction difficultly. As a result, polyester with high molecular weight and melting point is hard to be prepared only by melt polymerization.
(2) Azeotropy Method
The key to direct polycondensation for polyester synthesization is to exhaust water molecules. Only when most of the water molecules are exhausted can a product with a high molecular weight be obtained. An organic solvent which is not involved in the polymerization reaction and could dissolve the polymer is used in the polycondensation reaction, and it is processed with monomers and water for azeotropy and reflux reaction. Then, the reflux returns to the reaction vessel after dehydration and gradually takes trace moisture out from the reaction system and further propels the reaction towards the polymerization direction to obtain a product with high molecular weight. This is the method of azeotropic polymerization.
Currently, the method of solution polymerization for direct polyester synthesization is widely reported abroad. It could prepare a product with a high weight-average molecular weight and meet the need of practical application. Japanese Ajioka et al. have developed a technology of directly synthesizing polylactic acid through azeotropic dehydration, and weight-average molecular weight of the polylactic acid is over 300,000. Also, a series of aliphatic polyester compounds with weight-average molecular weight beyond 300,000 are synthesized directly with the same technology. By taking diphenyl oxide as a solvent, Zhao Yaoming et al. of China have prepared a polymer with a viscosity average molecular weight at 40,000 by solution polymerization.
However, disadvantages of such a method is attributed to the solvent introduced to the reaction system. The solvents with high melting points which are widely used at present such as dimethylbenzene, diphenyl ether, anisole and dibenzyl ether and the like are highly toxic, which do not only bring a bad influence to the environment, but also limit application of the polyester product prepared. In addition, massive use of the solvents has increased the production cost.
(3) Direct Polycondensation+Chain-Extension Reaction
Since direct polycondensation on monomers cannot prepare a product with a high weight-average molecular weight easily, people now are seeking a new way to obtain a polymer with a high weight-average molecular weight: to process an oligomer of the polyester prepared by direct polycondensation with a chain extender, thus obtaining polyester with a high molecular weight. Most of the substances serving as chain extenders are high-active molecular compounds with a bifunctional group or polyfunctional group.
Woo et al. adopted 1,6-hexamethylene Diisocyanate(HDI) as the chain extender, thus having increased weight-average molecular weight of polylactic acid to 76,000 from 1000. The reaction mechanism is as below:
Sepplala et al. also carried out chain extension reaction with bisoxazoline and 1,6-hexamethylene Diisocyanate(HDI), and weight-average molecular weight of the polylactic acid prepared is over 200,000.
The above method brings a problem to the application of polyester because of the introduction of toxic chain extender into reaction. Besides, crystallization capacity and speed of the polyester prepared therefrom decrease and crystallinity is lowered (or crystallization may even fail), therefore resulting in low melting point or no melting point at all.
(4) Solid-Phase Polymerization Method
Solid-phase polymerization method is a polymerization reaction conducted to a solid-phase oligomer at a temperature between the glass-transition temperature and melting point of the polymer. It can improve weight-average molecular weight of polyester polymer effectively and is only applicable to the polymerization of crystalline polyester.
Mechanism of solid-phase polymerization is depicted as below: gather the functional terminal group, micromolecular monomer and catalyst in an amorphous region by crystallizing low-molecular-weight polyester prepolymer (slice and powder or the like) to make the reaction equilibrium move positively; take the micromolecular by-product out from the reaction system by decompressing or utilizing inert gas so that the molecular chain could keep growing, and a product with a high molecular weight can be obtained. The solid-phase polymerization of polyester depends on chemical reaction and physical diffusion at the same time. Through reversible chemical reaction, the micromolecular product diffuses from inside of the particle onto surface of the particle and further diffuses into the decompressing atmosphere or inert gas atmosphere around. According to the principle of low-speed decision, the reaction rate of the entire polymerization reaction depends on the slowest step mentioned above. Time and temperature of polymerization, catalyst, flow rate of pressure or inert gas, crystallinity and geometrical shape of the prepolymer may influence the process of solid-phase polymerization reaction.
The catalysts for polyester polymerization which are released at present are mainly metal compounds such as alkoxide, acetylacetonate, oxide, complex, hydroxide or salt of organic acid of titanium, stibium, germanium, magnesium, calcium, zinc, ferrum, zirconium, lithium or manganese, etc. These catalysts are highly active in polymerization, but the activity of such catalysts also results in a poor thermal stability of the polyesters synthesized, especially to the polymerization of polylactic acid. Further, these catalysts may easily lead to side reaction such as racemization of lactic acid monomer.
Mitsui Chemicals disclosed a method for preparing polyester with volatile sulfonic acid as catalyst (CN99108012.2, JP2000-302852, JP2008-156665, JP2001-192443). A product with high thermal stability can be prepared in this method since no metal catalyst is used therein. Furthermore, sulfonic acid gradually volatilizes during polymerization so only few sulfonic acid catalyst is left in the polyester product. Therefore, the product features in high hydrolysis-resistance stability. However, found a problem with the method after research: the sulfonic acid catalyst volatilizes quickly at a high polymerization temperature, causing excessively low content of the catalyst in middle and later period of polymerization and sharply decreasing the polymerization activity. Consequently, the polymerization speed remains slow and polyester with a high molecular weight is hard to be prepared. On the other hand, though volatilization of sulfonic acid catalyst can be restrained by lowering the polymerization temperature, the polymerization speed also decreases because of the low polymerization temperature. Moreover, to reduce flow rate of the gas stream (flow velocity or gas stream) during solid-phase polymerization in inert gas can also control volatilization of sulfonic acid catalyst. However, it will prevent exhaustion of the by-product from the polyester condensation reaction and finally result in low polymerization speed. In all the embodiments of using single sulfonic acid in Mitsui Chemicals' patents mentioned above, the process of solid-phase polymerization requires at least 60 hours and weight-average molecular weight of the obtained linear polyester is lower than 150,000 without exception.
CN 200910004176.3 discloses a method for preparing polyester by using disulfonic acid or polysulfonic acid as a catalyst. Because the disulfonic acid or polysulfonic acid is hard to volatize or non-volatile, content of the catalyst during the entire polymerization process basically remains the same, and the system has extremely high polymerization activity, thus high-molecular-weight can be obtained. At the same time, nonuse of a metal catalyst endows the prepared product with great thermal stability. Yet a problem still remains in the method since the product prepared contains much highly acidic disulfonic acid or polysulfonic acid which is difficult to be removed in a subsequent process, thus resulting in poor hydrolysis resistance of the product.